1. Field of the Invention
The present invention concerns a method to generate a time raster-adapted measurement sequence for a magnetic resonance scanner (also abbreviated as an MR scanner) that can be executed in compatibility with the time raster of the magnetic resonance scanner. The invention thus in particular concerns translating predetermined time slices of a measurement sequence such that they can be executed in compatibility with the time raster of a magnetic resonance scanner.
The present invention also concerns a magnetic resonance scanner, and in particular programming of a controller therefor.
2. Description of the Prior Art
Magnetic resonance scanners today are a staple component of the clinical routine for examination of patients in hospitals. Magnetic resonance scanners also can be used for the examination of animals or biological samples. Magnetic resonance (also called nuclear magnetic resonance) as an imaging method was developed in the 1980s. Essentially, this method is sensitive to the mobility of water in tissue, for instance the human body.
The generation of magnetic resonance images in an MR scanner requires an exact, temporal workflow between the radio-frequency excitation of the spins, the spatial coding and the detection of the resonant response of the spin. The temporal workflow of the excitation, preparation and detection is called a pulse sequence or measurement sequence. The measurement sequence is composed of a series of time slices, each of the time slices having a specific length and at least one pulse with a pulse shape is associated with each time slice. Each of the time slices can be associated with a type from among transmission type for transmission of a radio-frequency (RF) pulse, and/or reception type to detect the resonant response of the nuclear spins as an RF signal, and warp type (WT) to prepare the nuclear spins. For the transmission type, differentiation is made between an excitation pulse to excite the nuclear spins and a refocusing pulse to refocus the nuclear spins. It is possible for a transmitted RF pulse to serve both to refocus and to excite the nuclear spins. In this context stimulated echo signals are produced by RF pulses both for excitation and for refocusing of the nuclear spins. A good introduction to echo creation is provided by Hennig, J. in “Echoes—How to Generate, Recognize, Use or Avoid them in MR-Imaging Sequences” in Concepts in Magnetic Resonance 3 (1991), 125-143.
Over the years a number of MR sequences or measurement sequences have been developed that serve very different purposes. For example, it is possible to significantly affect the contrast of an image by the suitable selection of a measurement sequence. The preparation of the spin system—for example by means of RF pulses, gradient pulses, wait times and so forth—thus has a decisive influence on the quality and property of the acquired magnetic resonance image.
The creation or the programming of the measurement sequence requires a high degree of expertise due to the sensitive connection between time workflows (also called timing conditions) and the individual pulses in the time slices of the measurement sequence. The design of measurement sequences has developed into its own field of MR physics, and a number of parameters that determine the quality of the image.
Today thoroughly complex MR sequences are used that, for example, enable images from the inside of the body to be acquired in real time, for instance of the beating heart. The programming of such measurement sequences requires the sequence programmer to take a number of conditions into account. In particular, a magnetic resonance scanner has at least one time raster in which events can occur (time slices with pulses begin and/or end, for example).
This means that all events within the measurement sequence are subservient to such a time raster in order to be executed by the MR scanner. Moreover, it is normally the case that an MR scanner has multiple, different rough time rasters: a more coarse resolution of raster for the gradient systems and a finer one for the radio-frequency systems (RF systems for short). Skill and a great deal of effort by a sequence programmer are required in order to make sure that pulses and events within a measurement sequence satisfy the narrow (physically predetermined) time conditions on the one hand and/or match the schedule of a magnetic resonance scanner.
Conventionally, the sequence programmer alone has made sure that timing conditions as they are predetermined by the physical behavior of the nuclear spins are maintained.